In the dock at the Nuremberg trials of 1945 and 1946: Hans Frank, born in Karlsruhe; once Adolf Hitler’s lawyer and governor general of Poland for the Third Reich, now charged with crimes against humanity for his part in the murder of three million people, including those in the death camps at Treblinka, Sobibór, Bełżec and Majdanek.

For the prosecution: Hersch Lauterpacht, who grew up in the Austro-Hungarian empire, near the city now known as Lviv in Ukraine, and who, after studying law in Vienna and London, went on to teach at Cambridge. He was a key figure in developing the idea of “crimes against humanity”, laying the foundation stones for international law and the modern laws of war. In his 40s, he was part of the British prosecution team at the trials of Frank and others.

There were strange connections between the two men, on opposite sides in the courtroom. The area in which Lauterpacht had grown up had been invaded by the Germans in June 1941. Lauterpacht was in England during this period and had been unaware that most of his family had been among the three million exterminated on Frank’s orders.

And there is this: as the trial proceeded, Lauterpacht would repair to listen to his favorite piece of music, from which he took inspiration for this onerous task: St. Matthew Passion (BWV 244b) by Johann Sebastian Bach. Meanwhile, Frank, in his cell, discussed with the prison psychiatrist and summoned up in his head – seeking not only solace but affirmation – St. Matthew Passion.

Each man heard in the great masterwork – in the same sublime solos and chorales – an entirely different, indeed contradictory, message; two opposed promulgations in the same score, two contrary cries in the same edifice of beauty.

Human rights lawyer Philippe Sands came across this unsettling coincidence while researching a book about his own family, to be published in 2016. He has now turned this compelling discovery, and other circumstances around the trial of Hans Frank (for example, also working for the prosecution was Raphael Lemkin, who had studied in Lviv – in the 1920s, when it was part of Poland and called Lwów – and had recently coined the term “genocide”. Sands believes that his family, too, had been obliterated on Frank’s orders), into a remarkable event to be staged at the end of this month by one of Britain’s leading young directors of opera, Nina Brazier. It combines narrative, which Sands will read with Vanessa Redgrave, images and music (from Bach via Ravel to Leonard Cohen), with Sands’s friend from childhood, French bass-baritone Laurent Naouri, accompanied by Guillaume de Chassy on piano.

Among the more extraordinary elements will be a “hymn” written to Hans Frank by Richard Strauss, the text for which appeared in a book by Frank’s son, Niklas, who has himself been involved in preparations for the performance. “We became immediate friends,” says Sands of Niklas, “after an initially strange handshake with the man whose father murdered my grandfather’s family, and three million other people.” There will also be never-before-seen film from the Kraków ghetto found in Niklas Frank’s family archive.

Philippe Sands is an Anglo-French lawyer with Jewish family origins around what is now Lviv also. He is best known for his books Lawless World, which detailed the illegality of Tony Blair’s war in Iraq, and Torture Team, about instructions for interrogation from US secretary for defense Donald Rumsfeld that led to calls for him and others to be indicted for war crimes. On a wall at his home beside Hampstead Heath in London hangs a map of the small town of Zhovkva, whence his great-grandmother came, from the same street as Hersch Lauterpacht.

Sands’s staging tells how Lauterpacht left for England, Lemkin for America and then converged home, in their way, to the trials at Nuremberg. (Sands’s own career has proceeded in the slipstream of the work of Lauterpacht and Lemkin.) Hans Frank was Adolf Hitler’s lawyer in cases dating back before the Führer’s rise to power, a loyalty for which he was appointed governor general of Poland in October 1939, to which he promised: “We bring art and culture.” He also guaranteed that “the Jewish problem will be addressed”.

Frank was a man of letters, a talented pianist and friend of Strauss. The remarkable book by his son is unprecedented in Holocaust literature (published by Alfred Knopf in America as In the Shadow of the Reich, but incomprehensibly refused by British publishers) and a monument to his German generation’s reckoning with the Shoah. It is a visceral challenge to the father, whom the text addresses in the second-person singular like an open letter, but also describes him thus: “You could play Chopin so beautifully. You loved Beethoven. You were friends with Richard Strauss.” At the end, as the Red Army rolls back the Wehrmacht’s conquests, “even in the chaos of battle, culture and the arts maintained their same important place on your scale of values”. Towards the end of the Reich, recalls his son, Frank was even chastised by Heinrich Himmler for “gluttonous and inappropriate behavior, with those theatre and opera performances of yours”.

In his engrossing book, Niklas quotes from his father’s diary: “This evening I was . . . at the great festival concert which [Wilhelm] Furtwängler conducted at the Philharmonic Hall for the German People’s Winter Benefit for the Needy.” Hitler, Hermann Goering and Joseph Goebbels were also there. “It was a powerful, profound, thrilling experience,” writes Frank Sr., “to hear this true giant of a conductor recreate the overture to Der Freischütz, Brahms’s Fourth Symphony and Beethoven’s Seventh. A magnificent evening of consecration. With indescribable emotion, I felt the years I have experienced pass before me, accompanied by this glorious music.”

So what did Hans Frank the war criminal and Hersch Lauterpacht who wrote the law on crimes against humanity hear in Bach’s St. Matthew Passion? “I do not stake any claim to expertise in music,” Sands says. “I’m an amateur enthusiast, and not even that when it comes to Bach. But I’ve read and thought a lot about this, and there is an interpretation whereby Bach used the passion of Matthew to register a Lutheran affirmation of the creativity of the individual against the Catholic commitment to the centrality of the group, the whole. It’s about the direct relationship between the individual and God in which, one may infer, Lauterpacht found an inspiration for his idea of individual human rights.

“I wonder about Frank,” muses Sands. “Whether he was an intelligent and cultured man, and if he was, did he really understand St. Matthew Passion? Frank may have heard in these glorious chorales an assertion of the collective, the rights of nation over those of the individual, which was the basis for his defense, such as it was. But there’s an irony here, because Bach writes in such a way as even the chorus sings in the first person singular – ‘ich.’ This is the individual celebrant communing with the deity, which opposes the Catholic idea of communion through celebration.”

In open court, Frank said: “I bear the responsibility” and: “I am possessed by a deep sense of guilt.” In his cell, Frank converted to Catholicism – to the derision of his son, whose book goads the father on his final pleas for divine mercy, for himself and family. Sands is less immediately dismissive: “It’s debatable why he converted, but having done so, he invokes the St. Matthew Passion as an affirmation of his conversion. Which is strange: he must have known that Bach and his intellect were Lutheran. I think Frank’s conversion was a strategic one and as such it reminds me of Blair. They were incomparable, of course, and we assume they converted in good faith, but in a way, Catholicism is the easy way out to self-absolution, and that is one of the things Bach takes issue with in this music.”

Sands explains how he and Laurent Naouri had to choose a section from the St. Matthew Passion to illustrate the obsession of each man. “For Lauterpacht I wanted Geduld, Geduld!” Sands says, “which means ‘patience’, but Laurent said it wasn’t in his range.”

“We chose Gerne will ich mich bequemen,” Laurent says: “‘Gladly would I, fear disdaining/ Cross and cup, without complaining’ – which is about determination and patience. And for Frank, it has to be Erbarme dich – ‘Have mercy . . . regard my bitter weeping’ – praying that he will live. I think he probably knows that he won’t [Frank was executed in 1946], but there is a part of him attuned to this music, some last feeling of hope.”

But did Hans Frank believe his own defense? If anyone can be the judge of that, it would be Sands, who has faced people in Frank’s shadow when he has acted for Human Rights Watch against General Augusto Pinochet, or cases in international courts involving Yugoslavia or Sierra Leone, and who now confides what everyone wonders about trial law: “When you are involved in a case, there’s a moment where you may come to believe your own arguments, however implausible. Frank was a courtroom lawyer too, and in a courtroom there can be this suspension of disbelief. I think Frank as a defendant may well have gone through that process, of not necessarily believing his own argument, but believing it enough to think it might get him off the hook. I don’t think he believed he was innocent, but I think he may have thought he was not sufficiently culpable to warrant the death penalty.” Sands befriended an interpreter from the trial, Siegfried Ramler, who told him: “I looked Frank in the face and thought, ‘This is a man who knows he has done wrong.’ He was looking for mercy, but not exoneration.”

Niklas Frank is a man of wit and charm; the dry humor that charges his book with bitterness makes his conversation agreeable. He became a war correspondent for Stern magazine and we were both in Iraq and former Yugoslavia, though we never met. I talk to him about finding concentration camps run by Bosnian Serbs in 1992 and he talks about “finding books, some in German, left in abandoned houses in empty Serbian villages” after the Croatian offensive around Knin during 1995. “There,” says Frank, “even the animals were dead. I felt like the loneliest man in the world.”

Niklas Frank was born in 1939 and, as he says: “My memories of the Third Reich were those of a child.” He recalls “a harmonium in the attic at our home in Bavaria, up a narrow staircase. I watched him play. He would improvise a great deal and the music always seemed sad to me, as though he knew what his end would be.”

Hans Frank “was so well educated,” says his son. “He was a close friend of Strauss; he knew every kind of classical music and he loved it. When Strauss discussed music with my father, he was on his own level.” In his book, Niklas recounts how such exciting movements as expressionism and naturalism “passed you by,” but in conversation relates how he found something unexpected in the programming of concerts sponsored by his father in occupied Kraków: “There was work there that was not just the usual cliche of Bach and Beethoven. There was modern music from the late nineteenth century and early twentieth century, which would have been considered decadent in Berlin – but this was Kraków. It’s there, being played, and I was astonished to find it.”

To perform such new music, Hans Frank had wanted as his programmer and concertmaster a man called Friedrich Franz Stampe, of whom Goebbels, who controlled the arts, disapproved, “but my father petitioned Goebbels and – amazingly – succeeded.”

The heating system at Laurent Naouri’s home in Paris, where he, Sands and Nina Brazier are rehearsing, has broken, so he has adjourned, as must our conversation, to a hotel on Boulevard Raspail. He gets directly to the point, without so much as a coffee between him and his argument: “Hans Frank was a great pianist; when he had to make a decision, he would play a while on the piano – something beautiful to help him with his thoughts. And it comes as a shock to think of Frank doing this – that he was a mass killer, but he had sensibility, a soul.”

“But the meaning of music is wide open,” Naouri explains. “Most music moves between tension and release, tension and release. Everything is built around the dominant seventh and the tonic, building pressure and the orgasmic conclusion. And this is primal, it affects all beings, whatever their ideology. Why would the same music move Lauterpacht and Hans Frank? Because they are both human beings. They both get hungry and both need food; they both experience these moments of tension that need to find rest. This has nothing to do with morality – it’s physical.”

I ask Naouri about Nazism and the cult of the sublime irrational. “What Bach composes in the St. Matthew Passion is rational,” he replies. “What is irrational is the feeling of sheer beauty that comes out of it. Not how the music was done, but why it does this to us.”

Niklas Frank writes in his book: “Here’s something I don’t want to suppress: a song for you, written by Richard Strauss. Yes, Strauss sang a song for you and accompanied himself on the piano. You were standing next to him, in his home, struck dumb in your vanity, flattered by the fact that this world-famous composer, for you alone, only you, had made up this little ditty and turned it into a song. You hum the melody and I’ll write the text: ‘Who enters the room, so slender and swank?/ Behold our friend, our Minister Frank/ Like Lohengrin sent by God, our master/ To save us all from every disaster . . . ’ What’s he thanking you for, Father? I have yet to discover that,” concludes the author. The song will never have been performed – apart from a dry run at the Hay Festival – until Sands’s event at the end of this month.

Niklas explains: “When my mother died, on my twentieth birthday in 1959, I said to myself, ‘I’ll work through all the papers and pictures, lots of stuff.’ Luckily she had always copied out letters she wrote to my father, by hand that is, words between her and her husband at the time of the Nuremberg trials.

“And suddenly, in the letters she is very excited, as if every danger to his life was passed: a Swiss newspaper had reported Strauss as having written this song to Hans Frank. And she was so excited: Strauss is now saving her husband! We will be free! She was so convinced that in another letter, she plans another child with him.”

Niklas Frank believes the song predates the war. “These are not stupid people,” he argues, “and it’s funny.” He recreates the scenario: “My father is passing by Garmisch, where Strauss lives, and telephones to plan a call at short notice: ‘I’m nearby, why don’t I drop by?’ Strauss thinks up a little welcome, a surprise for his friend. He writes his little song. My father opens the door and there is Strauss at the piano, singing this gay verse. It’s easy-going, an afternoon piece. I think it’s obviously during peacetime; such an occasion wouldn’t make sense during the war.”

The Frank family tried to ascertain whether any music survived, to be told by Schott, publisher of most of Strauss’s music, that it did not. Niklas Frank is unconvinced: “Strauss was a vain character and would never have thrown away even the smallest piece of music. I believe it has been hidden.” Sands was told “it has disappeared.”

But whether the music was lost or is hidden, it does not exist for performance at the Purcell Room. “Could we reconstruct it?” Sands asked Naouri. “We could try,” replied Naouri. “I wrote to a friend, Frédéric Chaslin, a composer, conductor and master of spoof,” he now recalls. “Chaslin brilliantly pastiches Debussy and I asked him to do the same with Strauss. Two days later, back came the score, the full kitchen sink, more Strauss than Strauss!

“It’s not black and white with Strauss,” says Naouri. “He was noted to be in favor of the regime, but he had Jewish relatives. He intervened with the government to keep Stefan Zweig as a librettist. He did what he could for the Jews he knew.” Quotes from his diaries register contempt for a “criminal . . . anti-culture regime”.

In January 1993, Vanessa Redgrave convened a rally at the Thalia Theater in Hamburg to mark the sixtieth anniversary of Hitler’s seizure of power, at which Holocaust survivors and deportees from Bosnia shared a platform, to denounce the gulag of camps in Bosnia, siege of Sarajevo and continued desecration by neo-Nazis of memorials built by Germany to Jews murdered during the Shoah.

Now, Redgrave and I sit outside a cafe near Twickenham Studios in London, while she smokes, drinks coffee and talks about these entwinements across history, of horror and defiance, that propel her work, and command her interest and involvement in Sands’s piece.

After a long opening conversation about Bosnia and the world today, she poses the question that pervades this work: “How and why is it possible that the Nazis knew classical music, and listened to classical music, while the crematoria were aflame? It runs though my head all the time.”

But Redgrave is a proud heretic and adds: “Is all this so far from what we’ve been talking about? How do those politicians in the UK sit in a concert hall and enjoy Beethoven while all this is going on? What do they think they are listening to after what they do?” Her eyes dart from behind spectacles, with righteous rage. “Mass murder was committed in Bosnia, with concentration camps, the torture of rape. Outrageous antisemitic acts of arson in Germany, in 1992/3, coincided with lethal attacks against Turkish immigrants. Then came Srebrenica, which was later defined as genocide. Were not European governments complicit?”

Then she invokes the defiance of art, and her profession, the kind of resistance that made for Rock Under the Siege, lunchtime concerts of chamber music and Waiting for Godot in Sarajevo: “I remember that both in Sarajevo during the siege, and in Kosovo, the artists and musicians played music and performed to strengthen survival and resistance. At all times music and theatre: to help civilians suffering intolerable hunger and war or the violence of occupation.” However, she adds: “The essential horror is that the perpetrators of war, siege, murder and occupation apparently also need, appreciate and promote music! To assist them in mass murder! Which is why I am glad Philippe has asked me to join him for A Song of Good and Evil.”

“I think,” posits Sands, regarding Redgrave’s last point, kernel of his piece, “that we find this unsettling because we want to hope that beauty, the notion of beauty, only makes us better people, that great art is good for humanity. This story shatters that illusion. It illuminates how bad people, too, appreciate beauty – and use it.”

Why, I asked Niklas Frank, could people such as his father love Bach, Beethoven, Schubert, music of such beauty? “I have never found an answer,” he replied.